Micro-Irrigation for Greenhouse Crops
Irrigation Emitters
The actual application of water in a micro-irrigation system is through an emitter that controls the flow of water from the lateral line into the substrate. The emitter decreases the pressure (reduces the head) from the lateral line to the substrate. This may be done by small holes, long passageways, vortex chambers, or other mechanical means. The quantity of water delivered from these emitters is usually expressed in gallons per hour (GPH). Emitters can be divided into two categories: line-source emitters and point-source emitters.
Line-Source Emitters
Line source emitters consist of drip tubing with supply orifices to meter water before it enters the line; then, the water passes through a labyrinth of flow paths to dissipate or compensate pressure and exits to one or more distribution orifices. Line-source emitters are suitable for closely spaced row crops, with the rows separated several feet apart, as with most vegetable crops. The volume of substrate irrigated by each perforation overlaps with that of the perforations next to it, resulting in a long, narrow block of irrigated substrate that surrounds the roots of the entire crop row Line source emitters use two main tubing configurations.
Thin-Walled Drip Tape
A thin-walled drip tape is a thin-walled polyethylene product, collapses when not pressurized, and has emitters formed into its seam during manufacturing (Figure 17.2). The thin-walled drip tape inflates upon pressurization. Drip tape systems can be applied to crops in beds where plants are growing at a specified spacing. Often, drip tape is used with high-value crops, such as strawberries, vegetables, flowers, or nursery stock. Drip tape can, in some cases, be used for more than 1 year in the greenhouse but is typically removed and discarded at the end of each growing season. Irrigation drip tapes are typically made of high-quality, thin-walled polyethylene (PE) tubing, often referred to as LDPE (low-density polyethylene) in a wide range of diameters, wall thickness, emitter spacing, and flow rates. Tape thickness is usually between 4 and 10 mils, though a thickness of up to 25 mils can be purchased. The emitter spacing is selected to closely fit plant spacing for most row crops.
Thick-Walled Drip Line
The thick-walled drip line is a robust variation of the thin-walled drip line (Figure 17.3). The internal emitters are molded or glued to the drip line. It is more durable because of its considerable thickness. The diameter of the drip hose is like that of the thin-walled drip line. Unlike the thin-walled drip line, the drip hose emitter spacing is wider, and it operates at a higher pressure. The emitter discharge ranges from 0.2 to 2 gallons per hour (GPH). A thick-walled drip hose is typically laid on the ground and retrieved at the end of the cropping season.
Point-Source Emitters
Point-source emitters are used when widely spaced point sources of water are needed, as in the case of greenhouse crops where they are spaced several feet apart. Point source-type emitters are attached to the lateral pipe. The installer can select the desired location to suit the planting configuration or place them at equally spaced intervals. Point source systems operate under somewhat higher pressures than line source emitters.
Non-Pressure Compensating Emitters
Non-pressure compensating emitters use a turbulent flow action, which provides greater durability and longevity along with clogging resistance and low maintenance. Non-pressure compensating emitters will have varying output flow at varying inlet pressures. Therefore, the flow will vary along uneven terrain, and each dripper will emit a different amount of water depending on its location on the supply line.
Pressure-Compensating Emitters
Pressure-compensating emitters, also referred to as PC emitters, deliver a precise amount of water regardless of changes in pressure due to long rows or changes in terrain (Figure 17.7). The main feature of these drip emitters is the ability to continuously adjust to varying water pressures, ensuring a constant flow rate from each emitter, regardless of water pressure fluctuations or varying elevations along the line.
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